Cellulose ester films have been used for supports of photographic negative films and for polarizing plates as films protecting polarizers used in liquid crystal displays, due to their high transparency, low birefringence properties, and easy adhesion properties to such polarizers.
Over recent years, the production volume of liquid crystal displays has remarkably increased due to their thin depth and lightweight and demands therefor are increasing. Further, TV sets using a liquid crystal display have features such as thinness and lightweight, whereby large-size TV sets have been produced, but such a large size has not been realized by conventional TV sets using cathode ray tubes. With this demands for polarizers and polarizer protective films constituting liquid crystal displays are increasing.
These cellulose ester films have been conventionally produced only via a solution casting method. The solution casting method is a film formation method in which a cellulose ester is dissolved in a solvent and the resulting solution is cast into a film. Then, the solvent is evaporated, followed by being dried to obtain a film. Such a film formed via the solution casting method exhibits enhanced flatness, whereby by use thereof, liquid crystal displays realizing high image quality without non-uniformity can be obtained.
However, the solution casting method requires a large amount of an organic solvent, resulting in the problem of a large environmental load. A cellulose ester film is formed using a halogen-based solvent having a large environmental load due to its dissolution characteristics, whereby especially, the reduction of the used amount of the solvent is being required, resulting in the difficulty of increasing production of a cellulose ester film via the solution casting method.
Therefore, in recent years, attempts for melt film formation from cellulose esters without any organic solvent have been conducted for sliver halide photography (Patent Document 1) or polarizer protective films (Patent Document 2). However, cellulose esters are polymers exhibiting very high viscosity during melting, as well as having high glass transition point. Thereby, it has become clear that to lower melt viscosity and glass transition point, a plasticizer is effectively added.
In above Patent Documents 1 and 2, used are phosphoric acid ester-based plasticizers such as triphenyl phosphate or phenylenebisdiphenyl phosphate. However, the results of investigations conducted by the present inventors made it clear that these phosphoric acid-based plasticizers exhibit high moisture permeability, whereby phosphoric acid esters are decomposed and phosphoric acid is generated, resulting in the problem of deterioration of a cellulose ester and a polarizer caused by the generated phosphoric acid.
As plasticizers used for cellulose esters other than phosphoric acid ester-based ones, sugar derivatives are disclosed in Patent Documents 3, 4 and 5. Plasticizers containing sugar derivatives exhibit enhanced chemical stability and do not generate any strong acid causing deterioration of cellulose esters, being, therefore, preferable plasticizers for the cellulose esters.
Generally, in production of a cellulose ester film, conveyance is carried out using conveyance rolls and drying is conducted using a drying apparatus or through a stretcher to obtain a cellulose ester film. However, it has become clear that in the methods described in Patent Documents 3, 4 and 5, foreign substances are deposited on rolls during the process using these conveyance rolls and these foreign substances are transferred to the film, resulting in problems such that the film surface is contaminated and pressure defects, in which the film is partially deformed via pressure by raised portions of the foreign substances, are produced.
Further, a cellulose ester film provided with a functional layer such as a hard coat layer, an anti-reflection layer, or an antistatic layer is commonly applied on the top surface of a display device such as a liquid crystal one. However, it has become clear that in the methods described in Patent Documents 3, 4 and 5, abrasion resistance of a cellulose ester film provided with such a functional layer is deteriorated.
When a polarizing plate is produced by bonding a polarizer protective film and a polarizer, a cellulose ester film is immersed in an alkali liquid of high temperature and high concentration to allow a water-soluble adhesive to be easily coated, namely, saponification is carried out for hydrophilic treatment of the film surface, and then the adhesive is coated for bonding to the polarizer. However, in the methods described in Patent Documents 3, 4 and 5, it has become clear that noted is the problem of adhesion properties of a cellulose ester film after saponification, resulting in difficulty in continuous production of polarizing plates; and also, when a cellulose ester film and a polarizer are bonded together, minute bubbles are generated at the interface therebetween, resulting in occurrence of minute foreign substance defects on the bonded surface. It has become clear that these defects impair visibility to a large extent and decease yield markedly in the production process of polarizing plates. Improvement of these problems is a big challenge and it has been demanded that these problems are solved.    Patent Document 1: Japanese Translation of PCT International Application Publication No. 6-501040    Patent Document 2: Unexamined Japanese Patent Application Publication (hereinafter referred to as JP-A) No. 2000-352620    Patent Document 3: Japanese Translation of PCT International Application Publication No. 2005-515285    Patent Document 4: JP-A No. 2006-265301    Patent Document 5: JP-A No. 2007-138121